甘肃北山火石山哈尔根头口布花岗岩年代学、地球化学及其地质意义

Geochronology and Geochemistry of Haergentoukoubu Granites in the Beishan Area,Gansu,China and Their Geological Significance

首次利用锆石LA-ICP-MSU-Pb法测得甘肃火石山东北部侵位于长城系、蓟县系以及志留系中的哈尔根头口布花岗岩形成于中泥盆世早期(同位素年龄为387.6±8.2Ma),同时发现该花岗岩体内存在晚蓟县世产物(继承性锆石年龄1172±7Ma)。哈尔根头口布花岗岩体主要由钾长花岗岩、二长花岗岩、花岗岩组成,岩石具有高硅(SiO2含量为70.28%～79.4%)、富碱(Na2O+K2O为6.52%～9.11%)、低铝(Al2O3含量为8.82%～14.05%)、低Sr(含量为40～150×10-6,平均为101.5×10-6)的特征、岩石的δ<3.3、A/CNK值为0.8～1.1,为钙碱性准铝质到弱过铝质岩石;稀土元素具有明显的LREE富集(LREE/HREE=2.89～16.4,LaN/YbN=1.82～25.18)和Eu亏损,相对富集Rb、Th、Ce和Sm,而亏损Ta、Nb、Zr、Hf。综合分析认为该花岗岩为A型花岗岩,产于构造挤压向构造伸展转换的地质背景之中。

LA-ICP-MS U-Pb isotopic dating for zircon separated from the Haergentoukoubu granites yields two ages： 387.6±8.2Ma and 1172±7Ma.These granites were emplaced in the Changcheng System,Jixian System and Silurain System in Huoshishan,Beishan,Gausu.The former probably represents intrusive age（early-middle Devonian）of the granites and the latter probably represents the age of late Jixian System inherited zircon inside the granites.Composed mainly of moyites,monozogranite,granodiorite,the granites are characterized by high SiO2（70.28%～79.4%）,and high Na2O ＋ K2O（6.52%～9.11%）,low Al2O3（8.82%～14.05%）,and low Sr（40～150×10-6,averaging 101.5×10-6）.Trace elements analysis shows that the average ΣREE is 168.52×10-6,the ratios of LREE/HREE and LaN/YbNare 2.89～16.4,1.82～25.18 respectively.The REE distribution patterns show that the granites are enriched in Rb,Th Ce and Sm,and depleted in Eu,TZ,Nb,Zr and Hf,with LREE/HREE=2.89～16.4,LaN/YbN= 1.82～25.18.The comprehensive analysis also suggests that the granites are metaluminous to slightly peraluminous calcicolous alkaline series rocks,which formed in the tectonic setting of transformation from collision to extension in earlier-middle Devonian.